The endogenous viruses of the guinea pig include the previously unclassified retrovirus, GPV, and two herpes viruses. A homologus RIA to GPV p26 was used to show that normal cells and animals do not express GPV or make antibody to the virus. Long term cell cultures and transplantable tumor lines express varying levels of virus, but there is no evidence that GPV may be an etiological factor in the transformation of guinea pig cells. Interspecies immunoassays have been developed which detect immunological determinants common to GPV p26 and MMTV p28, SMRV p35, or viper retrovirus p24, and it seems possible that MMTV, VRV and type D retroviruses may have had a common origin.